The present invention is directed generally to radiocommunication systems and, more particularly, to techniques for measuring a signal strength of a supervisory audio tone in such systems.
In cellular mobile radio systems it is desirable that a mobile station with an established connection on a radio channel should be able to maintain the established connection when moving from one cell served by one base station to another cell served by another base station. The process by which a mobile station maintains an established connection when moving between cells in a cellular radio system is generally called handoff. It is also highly desirous that a mobile station with an established connection on a radio channel be able to maintain the connection when moving within the same cell, even if the radio channel being used is subject to increased interference. If the quality of the established connection falls below specified parameters it is further desirable to automatically disconnect the connection in the event that handoff or other signal processing fails to improve the quality of the connection.
In general, radiocommunication is only possible when the desired information-carrying radio signals have sufficient signal strength at the receiver and are sufficiently strong in relation to noise and interfering radio signals at the receiver. The minimum signal strength of course depends on the particular features of the system, e.g., the kind of modulation and receiver used. In order to determine if an established connection should continue on a selected radio channel between a mobile station and a base station, handoff and disconnect processes perform various measurements on radio signals at the intended base and/or mobile stations.
The first cellular mobile radio systems in public use were analog systems used to convey speech or other analog information. These systems comprised multiple radio channels for transmitting analog information between base and mobile stations by transmitting analog-modulated radio signals. In general, the signal measurements made during the handoff and disconnect processes in such systems were performed by the base stations. One such system is known as the Nordic Mobile Telephone system NMT 450. Another known analog cellular mobile radio system, of particular interest as background to the present invention, is the Advanced Mobile Phone Service (AMPS) mobile radio system utilized in the U.S.A.
Recently, digital cellular mobile radio systems for public use have been designed. Digital cellular mobile radio systems provide digital radio channels for transmitting digital or digitized analog information between base and mobile stations using digitally modulated radio signals. Digital cellular mobile radio systems may offer substantial advantages, e.g. greater system capacity per unit bandwidth, over analog cellular mobile radio systems. To achieve these advantages there are certain demands. In particular, channel supervision, handoff and disconnect processes need to be performed rapidly and allowed to be frequently carried out relative to conventional analog systems.
In contrast to the introduction of digital-only cellular mobile radio systems, like the GSM system employed in parts of Europe, in areas with existing analog cellular systems it has been proposed to introduce digital cellular mobile radio systems which are designed to cooperate with the existing analog cellular mobile radio systems. In this way large legacy customer bases will not suddenly find that their terminal equipment has become obsolete. System designers of these hybrid systems believe that the digital portion of the system can be gradually introduced and, over time, the number of digital channels can be gradually increased, while the number of analog channels is gradually decreased. In order to provide complete compatibility, such dual-mode systems should comport with both the analog and digital standards that have been adopted, for example the analog AMPS and TACS standards.
In AMPS, a supervisory audio tone, abbreviated SAT, is transmitted on analog communication channels. More specifically, a base station transmits a SAT to a mobile station which receives the SAT and transponds the tone back to the base station to close the loop. The reason for transmitting the SAT in AMPS is that, in an interference-limited mobile radiocommunication network, there should be some mechanism for the receiving entity (e.g., a base station), to identify the transmitting entity (e.g., a mobile station) or at least with high likelihood exclude interchange of transmitter entities without the need for continuous transmission of a transmitter identity. Thus, the base station expects to receive the same SAT that it sent out, i.e., on the same frequency. If a different SAT is received by the base station then the connection is perceived to be interfered with and may be disconnected. In order to qualify as a valid SAT, it must be received by the base station at some predetermined signal strength. The AMPS standard specifies that the SAT need not be determined continuously, but should be determined at least every 250 ms. Thus, conventional analog base stations measured the SAT strength using analog hardware components and supplied a SAT signal strength report to the network periodically, for example the RBS 882 manufactured by Telefonaktiebolaget LM Ericsson reported SAT signal strength every 20 ms.
Although dual-mode base stations and mobile stations continue to support analog system functions, such as SAT detection, the ways in which these functions are supported are continuously being improved to reduce cost and improve quality. With the increased power of digital signal processors (DSPs), system designers are interested in implementing many signal processing techniques previously implemented using analog hardware components as DSP routines. DSP implementation has the added attraction of reducing the number of components, and hence the size, of the base stations and mobile stations.
Of course, digital signal processing also has its limitations. One design tradeoff which system designers confront when trying to implement analog signal processing techniques as DSP routines is that of speed of execution of a routine versus the amount of DSP resources, e.g., the number of millions of instructions per second (MIPS), which are allocated to execute the routine. Since digital signal processing is not yet so cheap that the number of MIPS used for each routine are insignificant, system designers are called upon to develop innovative digital signal processing techniques that reduce the number of MIPs used so that the digital signal processor can handle as many tasks as possible.